An image forming apparatus includes a plurality of process units which include a photosensitive drum, an electric charger, an exposing device, a developing device, and a transferring member. The electric charger charges the photosensitive drum. The exposing device emits light to the charged photosensitive drum to form a latent image based on image data. The developing device attaches toner to the latent image of the photosensitive drum to form a toner image. The transferring member transfers the toner image formed in the photosensitive drum to a print medium.
Positions of the toner images which are transferred from the plurality of process units to the transferring member and correspond to a certain point of the image data may be not matched on the transferring member. This is because there is generated a difference in slope of a region where the photosensitive drum and the transferring member abut on between the plurality of processing units, or a difference in slope of a region where the photosensitive drum is emitted with the light from the exposing device due to an attachment tolerance of the configuration of the plurality of process units.
Therefore, there is an image forming apparatus which includes an exposing device. The exposing device includes a plurality of light emitting element rows in a sub-scanning direction where a plurality of light emitting elements are arranged in a main scanning direction. The image forming apparatus corrects a difference in slope of each process unit by selecting a light emitting element to be turned on among the light emitting elements aligned in the sub-scanning direction. For example, if the image forming apparatus is configured to perform printing with four colors CMYK, the image forming apparatus selects a light emitting element to be turned on among the light emitting elements aligned in the sub-scanning direction in accordance with the position of the toner image of K color.
However, in the above method, the place where the light emitting element rows are switched is printed with a difference due to a resolution. As the place where the light emitting element rows are switched is increased, the number of differences is increased, and thus an image quality is degraded. In particular, if there is even one color causing a large difference, good image quality cannot be obtained.